Generator regulator



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WWA MA Sept. 17, 1940. P. H. CRAIG 2,214,879

GENERATOR REGULATOR Filed April 2. 1936 5 Sheets-Sheet 2 ,Tfr zzp. at

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Sept- 17, 1940 P. H. can@ 2,214,819

GENERATOR REGULATOR rma April 2, 1956 Vs sheets-sheet s Pamer H Cra/L59,

m, M @M Patented Sept. 17, 1940 UNITED`4 STATES GENERATOR REGULATORgalmerllunt craig,V Bethlehem, Pa., assigner to Inve'x Corporation,Cleveland, Ohio, a corporation o! New York Application ApriliZ, 1936,Serial No. 72,427 is'cwms. (ci. 1v1-119) The `present invention relatesto arrangements for regulating the voltage of ya generator, and inparticular to arrangements for the automatic regulation of analternating current generator.

An object of my invention is to devise a novel regulating means forincreasing the generated voltage in an alternator in response to anincrease in' the load on the alternator.

A further object is to devise'a regulating cir-w 10 cuitfor analternator which is -devoid of vibrating regulating contacts.

Further and more specinc objects will appear from the description of theinvention which follows.

1.5l My invention is illustrated in the accompanying drawingslinfwhich:y

Figure 1 isl a schematic vvcircuit diagram illustrating one form ofregulator according to my invention; l m Figure 2 is a graph showing aseries of curves for explaining the operation oi the regulator;

Figure 3 is a schematic circuit `'diagram illustrating a second form ofregulator according to my invention; y

g5 Figure 4 is a schematic circuit diagram-illustrating possiblevariations oi' my invention; and

Figure 4a is a circuit diagram showing a modification of the arrangementof Figure 4.

Referring to the drawings, the armature of an n alternator to beregulated is indicatedat i and the ileld winding ,of the alternator isshown at la. Exciting current is supplied to the neld la from theamature 2 of -an exciter generator through an acustable resistance I.The tleld 2a u of the exciter is supplied with current from the armature2 through a circuit which extends from `the positive terminal of the`armature 2, through a two-element thermionic vlvalve I, `through avariable resistance 5, neld'coil 2a and back to the negative terminal ofthe armature 2. .The iilament or cathode oi the tube I is energized fromsecondary winding Ba' of transformer I which is connected to a suitablesource of alternating current voltage. A transformer I has its primarywinding la connected in series with the load circuit of the alternator,and the outer terminals oi' the secondary winding 1b are conl nected to,the plate elements of a full-wave rectiner tube I. The mid-point .of thesecondary 50 winding 1b is connected to the terminal ot the exciterileld coil la which is connected to the negative terminal of the exciteramature 2;' and suitable connections are provided from the niament ofthe rectiiler tube I to the other terminal 5g oi the exciter eld windingIn, including thevv upper contacts associated with armature ta on aprotective switch 9, and the adjustable field resistance i. The illamentor cathode of the sistance lil is connected in shunt to the secy ondarywinding 'Ib of thetransformer l for the purpose to be described later,and condenser ii is connected in parallel with the exciter iield coil 2aand the adjustable field resistance 5. 10

The winding of the protective switch l is energized from the same sourceof current that energizes transformer 6, and while this source may be aseparate source of current,` I prefer to energize these elements fromthe 'output circuit of 15 the alternator, for example; by means of atrans-- former i2 connected to the alternator circuit and having asecondary winding supplying current to conductors il through a switchIl. The circuit for energizing the switch I is controlled by 4a.timedelay relay having a winding I5 connected across conductors IIthrough the lower -contacts associated with armatures 9c on the switchl. Arranged within the winding il is a -heat responsive core I6 havingone fixed end u connected to one of the conductors I3 and carrying atits free end the contact (a). The member it may be formed of aby-metallic strip which whenv` heated by a current ilowing throughwinding i5 curves in a direction to engage the .o contact (a) with thecontact (b)A and thereby complete a circuit through the winding ofswitch I. The core I6 possesses appreciable thermal lag and there willbe an appreciable time lag between the time of energization of thewinding u I5 and the time of closing contacts (a) yand (b).Y 'Ihe uppercontacts associated with armature Ic serve to complete an energizingcircuit through the winding of relay I independently of the contacts (a)and (b) upon operation of n switch l. The contacts associated with theamature Ib of switch I serve to short-circuit the anode elements of thetube I when the switch I is deenergized, and the lower contactscircuited by the amature lIa and the secondary winding of the currenttransformer I is shortcircuited by armature 9b. Under these conditions,the regulating circuit is inoperative, and the alternator will operateas an unregulated generator, the eld current for the exciter 2 beingsupplied entirely from the exciter armature. Upon closing the switch Il,the winding I5 of the time delay switch is energized through @armature8c, and the cathodes of tubes 4 and 8 are also energized through thetransformer 8. The switch 9 remains in the position shown in Figure 1until the time delay switch closes contact (a)-(b), and there issufficient delay in the closing of these contacts to permit the cathodesof tubes 4 and 8 to attain the proper operating temperatures before the.circuit of switch 9 is closed, thus'delaying the application of platepotential to the tubes until the cathodes reach proper operatingcondition. As soon as the switch 9operates, the short-circuit across theelements of tube 4 is removed, and armature 9a connects the cathode ofthe tube 8 to one side of the exciter field Winding 2a through theadjustable resistance 5. Also, the short-circuit across the secondary oftransformer 'I is removed, and, under these conditions, it will be seenthat the current transformer 1, in cooperation with the rectifier 8,supplies direct current to the exciter field winding 2a in parallel withthe exciter armature 2. Valve 4 is connected in proper direction topermit current to flow from armature 2 through the exciter field 2a, butit prevents current from flowing from the rectifier 8 through armature 2or through the alternator field la. At no-load condition, the entirecurrent flowing through exciter field 2a is supplied from the armature2, but as the alternator begins to supply* current to its load,additional field current is supplied to winding 2a by the rectifier 8.As the load supplied by the alternator armature increases, the amount ofadditional field current supplied to the exciter field Windingincreases, and the increased exciter field current causes a greaterexciting current to be supplied to the field winding of the alternator,and this in turn increases the generated voltage of thealternator tocompensate for the increased drop within the alternator and in the linesat higher load current values. It will be noted that the 'currentsupplied to the'exciter field coil 2a isin the same' direction as thecurrent supplied to the coil by armature 2. The polarity of therectifier is indicated by proper signs on opposite sides of condenserII.

The'operation of the regulator system may be better understood byreference to the Vgraph shown in Figure 2. The curves shown in Figure 2are not plotted from actual data, and are not intended to be drawn toscale, but the general shapes of the curves will indicate the generalcharacter of operation of the circuit. The lower lgroup of curves inFigure 2 show the variation in exciter field current with load currentunder certain conditions, while the upper group of curves show thevariation in the terminal voltage of the alternator with increasing loadcurrent under the same conditions as for the lower' group of curves.Curve A represents the exciter field currentwhen the switch Il is openand the regulator is inoperative. Under this condition, the exciterfield current remains constant regardless of any load currentvariations, and the terminal voltage of the alternator decreases as thelload current increases, as indicated by curve. A.

Curve B represents the exciter field current which is required tomaintain a constant voltage at the terminals of the ,alternator as theload current increases. The alternator terminal voltage curve under suchcondition is represented by the curve B.

Curve C represents the total exciter field current supplied jointly fromthe exciter armature 2 and from rectifier 8 when the regulator isoperative, as the load current increases in value. As will be seen fromFigure 2 this curve does not coincide with curve B, but it meets curve Bat full lo'ad current condition and at this point the exciter lfieldcurrent is of proper value to produce, the same terminal voltage of thealternator as for no-load condition.- For load current values less thanfull load condition, the exciter field current is greater than thatnecessary to maintain constant voltage, and the circuit isovercompensated over this range, as will be seen from the curve C'.

By adjusting the value of resistance I0, the slope oi' the curverepresenting the energizing exciter field current can be varied fromthat shown by the curve C in Figure 2, and the curve D illustrates acondition of adjustment where the slope has been reduced below that ofcurve C, and under this condition the curve D crosses the curve B atapproximately 75% full load current. Below this point the circuit isovercompensated, and above this point, it is undercompensated. Theterminal voltage curve corresponding to curve D is represented by curveD. It will be understood that the rectifier circuit including elements1, 8, I8 and II may also be designed and adjusted to increase the slopeof the exciter field current curve above that shown in curve C so thegenerator will be over-compensated throughout the entire range, and thiscondition is useful where it is desired to compensatel main differencesbetween the circuit of Figure 3 and the circuit of Figure l are: (l) Ahalf-wave rectifier tube 8a is employed instead of a full wave rectier,although a full wave rectifier may be used, if desired. (2) A relay I1is provided with an amature Ila which short-circuits a portion of theexciter field resistance 8a. when the relay is deenergized, and a secondamature IIb arranged to short-circuit a portion of the generator fieldresistance 8 when the relay is energized. This relay is controlled by acurrent responsive relay I8 energized from a series transformer Ilconnected in series with the alternator load circuit. (3) A relay 2lconnected across the alternator circuit by means ofV a switch 2lcontrols the circuits for energizing protective relay 8 and filamenttransformer l. (4) The time delay relay .for controlling the circuit toprotective relay 8 is of different construction from that shown inFigure 1 and vconsists of`a transformer 22 f' having a thermallyresponsive element 28 connected across its secondary and is .adapted tobe heated by current from the secondary winding.v It will be understoodthat the details `of the time'delay relay are not important, and anydesired construction may be employed.

The operation of the arrangement shown in Figure 3 is as follows: Solong as the switch 2| sei'ted lett the position indicated at 5b, betweenis open, or so long as the voltage of the alternator isbelow a valuesufficient to operate relay 20, relay 9 is deenergized and valve 4 isshort-circuited through amature c, while the'clrcuit through rectier 8ais open circuited at armature 9b. So long as the generator does notdeliver substantial current to'the line, relay I1 remains deenergizedand part of the exciter field resistance 5a is short-circuited. Underthe conditions stated above and as .shown in Figure 3, the alternatorwill operate as an unregulated generator. As soon as the voltage of thealternator increases to `a value sumcient to operate relay this relayoperates and closes the energizing circuits for filament transformer 5and the time delay relay controlling switch 9. The lower armature ofrelay 20 closes the circuit to transformer 6. 'I'he circuit through thetime delay relay extends from one side of the alternator circuit throughthe upper armature on relay 20, through armaturedc on switch 9, throughthe primary winding of transformer 22 and back to the other side of thealternator circuit. Current induced in the secondary of transformer 22.

flows through the thermally responsive element 23 to heat the same. and,after a predetermined period, the element 2l snaps from the positionshown into engagement with theupper contact 23a. and this completes acircuit through the winding of switch 9 extending from one side of thealternator circuit through the upper armature of switch 20, contact 23a,element 23, winding 9, and back to the other side of the alternatorcircuit. The switch 8 now operates, and the armature !c completes acircuit for maintaining the switch winding energied independently of thetime delay relay; armature 9b,closes the circuit leading from thesecondary winding of the 'current transformer 1 to the plate of therectiner 8a, and amature 9c removes the short-circuit from around valve4. lAs the load upon the alternator increases from zero towards fullload condition, the operation of the regulator is the same as thearrangement shown in Figure 1.

In order to prevent the voltageof the exciter from rising too high inthe upper range of load currents, I adjust the relay Il so that itoperates at load current values of one-half fuliload or greater, andtheoperation of this relay closes the circuit of relay I1 whichshort-circuits a P01'- tion of the generator eld resistance 3 andinserts an additional resistance 5a in the circuit of the exciter field.By this arrangement, the necessary range of voltage variation of theexciter is reduced, It is obvious that the relay I1 might be operateddirectly from current transformer I9 and relay I5 may be omittedi; Inoperating the circuit of Figure 3, the switch 2l may remain in closedposition during starting, since the relay 20 will not pick up before thealternator voltage has reached a definite value.

In Figure 4 I have shown a circuit diagram illustrating several possiblevariations in the regulator arrangement. Here also the general circuitarrangement is the same as shown in Figures 1 and 3, and correspondingparts are indicated by corresponding reference numerals.

In the arrangement illustrated in Figure 4, the protective relaycircuits have been omitted. since these circuits, while desirable, arenot essential. In Figure 4 I have also shown three different positionsfor the variable resistance which controls the exciter field currentsupplied from the exciter armature. The preferred position for thisresistance is shown at l, but it may be in-v use a transformer which theexciter armature 2 and the valve 4, or it may be inserted at 5c where itcontrols not only the exciter field current but also the eld currentsupplied to the alternator.

In order to obtain a closer agreement between the curve C and the curveB in Figure 2, Imay use two current transformers l and 1 connected inseries, as shown in Figure 4, with the secondary windings of thesetransformers connected in opposed relation. Also, one of the trans-`formers, for example, 1', is of smaller capacity than the othertransformer and is designed to become saturated at relatively low valuesof load current while the other transformer does not become saturatedeven at full load current. With this Iarrangement, the voltage oftransformer 1' may buck-out any desired percentage of the voltage oftransformer 1 at low values of load current, but as the load, currentincreases in value, and the transformerll `becomes saturated, thevoltage of transformer 1 will increasingly predominate over the voltageof transformer '1' and will produce a curve Q which .more closelyapproximates the curve B; In order to obtain a still closerapproximation, to the ideal curve, I may connect in shunt to thesecondary winding of transformer l a resistance element 24 having anegative resistance characteristic'such that its resistance decreases asthe current flowing therein increases. With such an arrangement, atlowload current values, the resistance of element 24.

is high and the full voltage of the transformer 1' is effective inopposing the voltage of transformer 1, but as the load currentincreases, the resistance of the element 24 drops and reduces theterminal voltage below the value which it would assume if the resistanceelement were not present. Resistance element 24 may be formed of vagaseous conduction tube, such as a mercury vapor tube, or of resistancematerial commonly known as "Thyrite," or both a mercury vapor tube and aThyrite resistance element may be connected in series across thesecondary winding of transformer 1'.

As illustrated in Figure 4, I may also provide a grid control element'25 for the rectier tube 8a, and this grid control element may besupplied with a biasing potential which varies in accordance with thevoltage variation in the alternator load circuit. For this purpose I mayis connected to the alternator circuit through a relay or amplifier tube2 and the switch 2t. A potentiometer 29 and a condenser .30 areconnected in series across the secondary winding of transformer 26, andthe grid 25 is connected to the variable contact of potentiometer 29.The values of resistance 29 andl condenser 30 are so chosen that thepotential applied between the cathode and the grid 25 has a laggingphase relation with respect tothe potential applied between the cathodeand the anode of the rectifier 8a, and the preferred amount of lag is ofthe order of 135 degrees. This condition of adjustment'rendersroperationof the rectiiler 8a dependent upon the value of the potential applied tothe grid 25 so that the greater the potential applied to the grid 25,the smaller will be the plate current flowing through the rectier tube,and vice versa, all as more fully explained ir'imy U. S. Patent2,001,836.

For bt operation it is desirable that the voltage changes supplied tothe transformer 25 be amplified above the percentage change of thealternator voltage. For this purpose I provide an arrangement forimpressing a variable biasing potential upon the grid of the amplifiertube 21. This biasing arrangement consists of a rectifier 3i supplied from the alternator circuit through a transformer 32, and this rectifiersupplies current to a biasing resistance element 33. The cathode of thetube 21 is lconnected to a variable contact on resistance 33. A circuitis also provided for supplying current to a biasing resistance 34 fromthe rectifier 3| through a ballast resistance 35, and a gaseousdischarge tube 33 (such as neon lamp) is connected across the terminalsof resistance 34 to maintain the potential drop across this resistancesubstantially constant and' independent of voltage Vvariations of thealternator. The polarities of the potential drops across resistances 33and 34 are as indicated in the drawings (in opposing relation), and themagnitude of these drops are so adjusted that for a decrease inalternator potential below normal, the biasing potential appliedto thegrid of tube 21 is increasingly negative or is less positive, and uponincrease in alternator voltage above normal, the biasing potentialbecomes less negative or more positive. It will thus be seen thatvoltage changes impressed upon the transformer 2i will be accentuated-over the changes in the alternator voltage. It will be understood thatthe voltage drop across resistance 33 varies in accordance with thealternator voltage variation, whereas the voltage drop across resistance34 is substantially constant. A resistance element 31 is connected inparallel with the primary of the transformer 26, and the value of thiselement is adjusted so as to causelmost of the platencurrent of .tube 21to flow-through this resistance. This arrangement prevents anysubstantial change in phase relation of the current supplied to thetransformer 23 as the resistance of tube 21 is varied.

So long as the switch 23 controlling thegrid Figure 4 is as describedabove for Figures 1 and 3. Upon closing the switch 23, the operation ofthe rectier 3a issubiect to control according to the voltage existingacross the alternator circuit. Should the voltage across the alternatorcircuit increase in value, the increased potential applied to the grid23 causes less current to flow through the rectier 3a and, therefore,reduces the amount of current ilowing through the exciter ileld winding2a. Similarly should the voltage of the alternator circuit decreasebelow normal, the voltage applied to the grid 23 permits a largercurrent to ow in the rectiiler 3a and to thereby increase the excitationof the exciter. It will, therefore, be seen that with the grid controlcircuit of Figure 4 operating, the regulator shown in this circuitcompensates both for voltage drop due to load variations and forvariations in voltage of the alternator circuit. Y

In Figure 4a I have shown a modified arrangement for supplying biasingvoltage to the grid of rectifier 3a in Figure 4. In this arrangement atransformer is provided with a primary winding 33 which isconnectedacross the alternator load 36. A secondary winding 43 isconnected to supply direct current to the plate circuit of 'a voltageampliner tube 21a through a rectiiier 4i,and the output circuit ofamplifier 21a is connected to the input circuit of ampliner 21 through acoupling resistance 31a. A third secondary winding 42 supplies directcurrent to the plate circuit of amplifier 21 through rectiiler 43, and afourth secondary winding 44 is connected in series with the platecircuit of tube 21 for supplying an alternatlng current component to theplate circuit. The output circuit of amplifier 21 is connected to thegrid transformer 2l in the same manner as in Figure 4. The maindifference between the arrangement of Figure 4a and that shown in Figure4 is the insertion of the voltage amplifier 21a. between the voltagebalance and the ampliiler 21. The presence of this amplifier makes itnecessary to reverse the polarities of the resistance elements 33 and 34as shown in the drawings and the amplitudes of the voltage drops acrossthese resistance elements isV adjusted so that normally the drop acrossresistance 33 is larger than the drop across element 34 and a negativebias is applied to tube 21a. Upon a drop in the line voltage belownormal value, the bias voltage applied to tube 21a becomes lessnegative, thereby increasing the plate current and applying a morenegative bias to tube 21, which in turn reduces the plate current nowthrough tube 21 and, therefore, reduces the potential applied to thegrid of rectiiler 3a. An increase in line voltage above normal operatesin reverse order to increase the potential applied to the grid 25 of therectiner 3a and thereby reduce the excitation supplied to the exciterileld circuit. A voltage stabilizer or regulator of well knownconstruction may be inserted between primary winding 33 and the loadcircuit, if desired, but in this case a separate transformer must beused to supply the rectifier 3l directly from the line.

,In all of the arrangements described herein, the exciting currentflowing through coil 2a is supplied from two sources connected inparallel, that is, from the exciter armature 2 from the line currentrectifier?A 3 or 3a. The uni-directional conductive elements 4 and 3prevent the interchange of current between these two sources. It isobvious that instead of supplying part of the exciting current from thearmature 2, a separate source of current may beprovided if desired.

In both Figures 1 and 3 the protective relay'3 is arranged to normallyshort-circuit the valve 4 and to open circuit the rectiiler 3 or 3a. Instarting up the alternator, the regulator circuits are inoperative andthe exciting circuits are according to standard practice for lanunregulated alternator. Since the relay 3 is energized fromthealternator circuit, the regulator circuits do not become effective untila predetermined voltage has been established across the alternatorcircuit, and, conversely, the regulator circuit is automaticallydisabled upon the failure of voltage across the alternator circuit.

In any of the circuit arrangements disclosed herein in which atwo-element thermionic valve is used as a rectiiler or as aunidirectional conductive element, the tube may be replaced by wellknown rectiiier'units of the solid-element type. It is also apparentthat greater flexibility of adjustment may be obtained by connecting therectiiler tube 3 or 3a to a variable contact on a resistance I3 alsshown in Figure'4. While I have shown the regulating rectifier 3 or 3aconnected to supply current to the field coil of the exciter, it isobvious that the rectifier could be connected in parallel with thegenerator ileld coil la, in which case the valve 4 would be insertedbetween the armature 2 and the variable .resistance I in order toprevent the rectifier I from supplying current through the exciteramature In such an arrangement, the exciter field would be connected inshunt to the amature 2` in the usual manner.

Various other modiiications falling within the scope of my inventionwill be obvious to those skilled in the art. 1

1. In combination.' an alternator having a load circuit. means forexciting said alternatorv in,-

cluding an exciting coil, means including a uni' directional conductivedevice for Nsupplying direct current to said exciting coil variable inaccordance with theload current flowing in the load circuit of saidalternator, a separate source of direct current, and circuit connectionsfrom said separate source including a uni-directional conductive devicefor normally supplying a substantial portion of the exciting current to-said exciting coil in the same direction as said firstl means and inparallel therewith.

2. In combination, an alternator having a load circuit, means forexciting said alternator including an exciting coil, a transformerconnected in series with said load circuit, a rectiiierk connected to asecondary winding of said transformer and connected to supply directcurrent to said exciting coil, a5 separate source of direct current, andcircuit connections from said separate source including auni-directional condnc`` tive device for normally supplying asubstantial portion of the exciting current to said exciting coil in thesame direction as said rectifier and in parallel therewithu 3. Incombination, a generator having load circuit, means for exciting saidgenerator including an exciting coil, means for supplying excitingcurrent to said coil to x the no-load voltage of said generator, meansconnected in parallel with said coil forsupplying additional excitingcurrent variable in accordance with the current flowing in the loadcircuit of said generator, and a uni-directional conductive deviceincluded-in series with each current supply circuit for pre,- ventingtheinterchange of current between said sources.

4. InL combination, an alternator having a field winding and a loadcircuit, an exciter generator for supplying exciting current to saidfield winding' and having an exciting winding, circuit 'connections forsupplying current to said exciting winding from the armature of saidexciter, a current transformer connected in the load circuit of saidalternator, a rectifier connected to the secondary o! saidtransformenfconnections from said rectifier to supply current to saidexciting winding in parallel with said exciter armature, and auni-directional conductive device connected in series with said exciterarmature for preventing current from said rectifier from flowing throughsaid exciter armature.

5. In combination, a generator havingv a field winding and a loadcircuit, an exciter generator for supplying exciting current to saidfield winding and having an exciting winding. means for supplyingcurrent to said exciting winding vari-y able in accordance with thecurrent flowing ixr the load circuit of said generator, and -meansresponsive to a predetermined current flowing yin said load circuit forsimultaneously increasing the resistance of the exciter field circuitand decreasing the resistance of the exciter armature circuit.

8. In combination, an alternator having a ileld winding and a loadcircuit, an exciter generator v for supplying exciting current to saidfield winding and having an exciting circuit, and means responsive to apredetermined current owingein the load circuit for simultaneouslyincreasing v,the resistance of theexciting circuit of said excitergenerator and decreasing the resistance of the exciting circuit for saidalternator.

'7. In a regulator, the combination of a generator field coil, twosourcesl of voltage for supplying current to said coil in parallel, athermionic valve connected between one source and said coil to preventcurrent from they other source from flowing therethrough,-a circuit forenergizing the cathode of said valve, means for normallyshort-circuiting said valve, and time delay means responsive to theenergization of said cathode circuit for removing the short-circuit fromsaid valve.

8. In combination, an alternator having a load circuit, means vforexciting said alternator in-A cluding an exciting coil, an energizingcircuit for said coil including a uni-directional conductive device,means for normally short-circuiting said device, and means responsive toa predetermined voltage in said load circuit for removing saidshort-circuit.

K 9. In combination, an alternator having a load /clrcuit, means foiexciting said alternator including an exciting coil, an energizingcircuit for said coil including a thermionic valve having a cathodeenergized from said load circuit, and means responsive to failure ofvoltage in said load circuit for short-circuiting said valve,

10. In combination, an alternator having a load circuit, means forexciting said alternator including an exciting coil, a rectifier forsupplying direct current to said coil from said load circuit, means fornormally maintaining the circuit of said rectifier open, an energizingcircuit for supplying exciting current to said coil independently ofsaid rectifier including a uni-directional conductive device, means fornormally short-circuiting said device, and means responsive to apredetermined voltage in said loadcircuit for removing thelshort-circuit on said device and for closing the rectifier circuit.

l1. In combination, an alternator fhaving a load circuit, means forexciting said alternator including an exciting coil, means for normallysupplying a magnetizing current to said exciting coil to iix the no-loadvoltage of said alternator, a transformer connected in series with saidload c circuit, a rectifier connected to a secondary winding ofsaidtransformer for supplying additional direct current to said coil inthe same direction as the normal magnetizing current but variable inaccordance with the current in said load circuit, and means vfor varyingthe resistance of the rectifier circuit in accordance with variations involtage of said load circuit. i

l2. In combination, an alternator having a load circuit, means forexciting said alternator including an exciting coil, means for nbrmallysupplying a magnetizing current to said exciting coil to nx the no-loadvoltage of said alternator, a transformer connected in series with saidload circuit, a rectifier connected to a secondary winding of saidtransformer for supplying additional direct current to said coil inthi.i same direction as the normal magnetizing current but variable invaccordance with the current in said load circuit, said rectifiercomprising a thermionic valve provided with a control electrode, and anenergizing circuit for said control electrode connected across said loadcircuit for varying the resistance of said valve in accordance withvariations in voltage of said load circuit.

13. In combination, an 'alternator having a load circuit, means forexciting said alternator including an exciting coil, a-pair oftransformers having their primary windings connected in series with thealternator load circuit, one of said transformers being of smallercapacity than the other and being adapted to become saturated in thelower range of load current values, a secondary circuit including inserial circuit relation, a secondary winding on each of saidtransformers, a

- cuit including in serial circuit relation a secondary winding on eachof said transformers, a

rectifier unit and said exciting coil, whereby "direct current issupplied to said coil from said transformers, the secondary winding onone of said transformers being connected in opposition to the secondarywinding on the other transformer and having a resistance elementconnected in shunt therewith having a negative resistance characteristicwhereby'the voltage of the other transformer increasingly prdominatesoverthe voltage of the shunted transformer as the current in said loadcircuit increases.

l5. In combination, an alternating current circuit having a variablecurrent owing therein, a pair of transformers having their primarywindings connected in series with said circuit. one of said transformersbeing of smaller capacity than' the other and being adapted to becomesaturated in the lower range of current values in said circuit, asecondary circuit including a secondary winding on each of saidtransformers connected in serial circuit relation, said transformersbeing-connected so that the voltages in said secondary windings are inopposing relation in said secondary circuit, the secondary winding ofsaid smaller transformers having a voltage at current values below thepoint of saturation not greater than the voltage of the secondary windning of the larger transformer, and a negative rcsistance elementconnected in shunt with the secondary winding of the smaller transformerwhereby the voltage of the larger transformer increaskv inglypredominates over the voltage of the shunted transformer as the currentin the primary circuit increases.

sPrimula. HUNT CRAIG.'

